Unicron is supposed to be the reason for all the different timelines as I understand it. However, his destruction did not eliminate those timelines. The universe was still in a state of chaos due to the black hole.

If one has an imagination, one can pretend closing the black hole also closed with it all the other universes except the one we are being presented with next year.

It's incredibly extreme, but it's also incredibly exciting and a good way to truly get behind the movie. The simple fact is, this movie will represent the entire fandom to the masses. We must support it